1. Field of the Invention
The present invention relates to a method of making a plasma display panel, and more specifically, a method of forming a dielectric layer on an upper substrate of a plasma display panel.
2. Description of the Background Art
A plasma display panel (hereafter, PDP) has a matrix structure which can express full color display by using a fluorescent material. Typically, a surface discharge type PDP has upper and lower substrates between which sustain and address electrodes are arranged in a matrix form. Discharge cells are divided by barrier ribs. In the discharge cells, ultraviolet rays are generated from plasma radiation of He—Ne and Ne—Xe gases. The ultraviolet rays stipulate the fluorescent material. When the fluorescent material transit from excited state to ground state, radiation occurs by energy difference between the excited and the ground states. In this way, display is achieved.
The manufacturing method of PDP comprises processes of making the upper and lower substrates and packaging them. The manufacturing method further comprises a process of forming the dielectric layer on the upper substrate.
According to the conventional process of forming the dielectric layer, the method of forming the dielectric layer comprises steps of: printing by screen printing technique which is repeated at least 5 times; drying; and firing which is repeated at least 2 times. Since the conventional method includes many steps, cycle time of the overall process became very long.
Further, according to the conventional process, mesh mark of screen mask which is generated during screen printing step remains even after firing. Therefore, the surface roughness of the final product decreases.
And, because the PDP passes through a plurality of printing and drying steps, the thickness of the dielectric layer is not uniform.
In order to solve these problems, the dielectric layer is made of green sheet, which is also called green tape, by using the tape casting device.
FIG. 1 shows a section view of an upper substrate of a PDP according to the background art. As shown in FIG. 1, the upper substrate of the PDP comprises glass substrate 10; a plurality of sustain electrodes 11, 12, 13, 14, which comprises ITO (indium oxide In.sub.2 O.sub.3 and tin oxide SnO.sub.2 semiconductor) film formed on the upper substrate; a plurality of bus electrodes 21, 22, 23, 24, 25, 26, 27, 28 which is formed on the sustain electrodes; and a dielectric green sheet 30 which envelops the sustain electrodes and bus electrodes and is formed on the glass substrate.
As this dielectric green sheet 30 is laminated on the sustain electrode and bus electrode of the glass substrate 10 and fired, the characteristics of the dielectric of the upper substrate are very good such that the surface roughness is less than or equal to 500 Å, the tolerance voltage is more than or equal to 5 KV, and the thickness uniformity is in ±1 μm.
As, by using this green sheet, the working process becomes very simple and the cycle time becomes short, the manufacturing cost decreases.
However, in this method of forming dielectric layer by using green sheet, the strength of the green sheet must remain over than some value in order to treat the green sheet easily in the process which the green sheet is laminated on the upper substrate. The reason is because it is possible to prevent the laminating badness by extending of the green sheet and the badness of the dielectric layer thickness after firing only if the strength of the green sheet must remains.
However, these green sheet of unique layer discolors the sustain and bus electrodes, decreases the penetration ratio, and generates some cracks on the MgO layer which is formed on the green sheet.
Recently, in the forming of the bus electrode, the printing method of Ag-paste is used rather than the conventional vacuum plating of Cr—Cu—Cr. However, in the printing method of Ag-paste, there is edge curl on the bus electrode which is the phenomenon that the side of the bus electrode is rolled during the processes of exposuring to light, etching, and firing.
FIG. 2 shows a picture of section of upper substrate of PDP according to the background art which is photographed by 20 magnification. As shown in FIG. 2, the bubbles are generated on the region where the dielectric contacts to the glass substrate and the electrodes.